Preserving the banner, which hangs in the Museum of American History on the Mall, could cost up to $15 million. And experts, anxious to make the right moves as they restore the flag, haven't even decided yet how to proceed.

``I think it's our duty to make it available to people for as long as we can,'' said associate museum director Ron Becker, in charge of saving the flag.

Visitors now see the flag as they enter the museum's main door. It hangs from the ceiling, stretching 40 feet down a wall - the size of a four-story building.

The banner had to be big for composer Francis Scott Key to spot it from the deck of a small storm-tossed sloop, rolling in the Patapsco River nine miles away, in the dawn's early light of Sept. 14, 1814. Key got real help from ``bombs bursting in air'' - a new invention of the time.

The flag still flew from its staff after an all-night British naval bombardment of Baltimore's Fort McHenry during the War of 1812, signifying the fort remained in American hands.

Now, unless the flag is taken down and subjected to extended high-tech preservation, it may crumble.

The flag's natural fibers have been inevitably damaged by air pollution and decay, said Suzanne Thomassen-Krauss, the museum's authority on textiles. The 15 stripes and blue field are wool, and the stars - two feet across, point to point - are cotton. Sewn with linen thread, the flag had cost $405.90 by the time Baltimore widow Mary Young Pickersgill and her 13-year-old daughter Caroline finished it a month before the attack.

The flag used to have a huge plain cloth hung over it, raised every hour so visitors could see it as the national anthem played. But conservationists decided the cloth gave little protection from visitors' breath and fibers in the atmosphere, so the flag is now on permanent display.

The Star Spangled Banner also got a light vacuuming in 1982, but nothing like the effort now planned, said Thomassen-Krauss.

``We used a vacuum with very low suction, so as not to remove any of the original fiber,'' she said. ``It just took off the same light dust you'd find on your furniture.''

Last November, Becker assembled about 50 conservators who met for two days, then broke up into groups available for consultation. Each deals with a part of the problem - how to take down the flag without damage, how to build a case for it, how to control the environment inside.

``It's a community we can go back to,'' Becker said.

Many problems still need discussion.

It's uncertain just how much the flag weighs. Becker estimated 125 to 175 pounds.

Building a case would require about seven tons of glass and would be a much more complicated job than fashioning a display case for a small document like the Declaration of Independence.

Experts could decide glass is not the right material because it scratches too easily.

Becker won't estimate how long the job will take but said he will try to keep the banner visible as work continues. By comparison, it took museum conservators more than a year to restore ``Old Glory,'' a flag only 17 feet long.

Researchers also are taking the opportunity to learn more about the Star Spangled Banner itself. In the years, before the museum got it in 1907, souvenir hunters snipped eight feet from its length.

And someone sewed a red V on one of the white stripes. ``We're researching who did that, and why,'' Becker said.